Weather Service Report


859 
FXUS66 KPQR 032332
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon 
331 PM PST Sat Dec  3 2016

.SYNOPSIS...A modest westerly flow aloft will keep a few showers over
southwest Washington and northwest Oregon this afternoon, with snow
in the Cascades. A strong cold front will push into southwest
Washington and northwest Oregon tonight then move south through the
area on Sunday for more rain and mountain snow. Expect heavy snow in
the Cascades, affecting travel through the passes. Snow levels will
drop into the coastal mountains and the Cascade foothills mid day
Sunday but precipitation will be dwindling, limiting any
accumulations. The next system will be approaching from the northwest
later Sunday night and move through the area on Monday. Expect more
accumulating snow in the mountains. Snow levels will drop close to
the valley floors Monday morning with some light accumulations
possible but not certain. Some precipitation will linger into early
Tuesday then start to dry out. Rather strong east winds are expected
to develop Tuesday night and Wednesday, setting the stage for more
possible winter weather Wednesday night and Thursday.
&&

.SHORT TERM...Tonight through Tuesday...A westerly flow aloft
combined with some modest onshore flow is keeping a few showers over
the forecast area this afternoon. The snow level will flirt with the
Cascade passes today. 

A large shield of moisture and clouds with the next system is
increasing off the coast as this system approaches the coast. This
front will spread in tonight with increasing rain and mountain snow.
The cold front will be quite strong with strong cold advection behind
it. The front will move south through the forecast area Sunday. Look
for heavy snow in the Cascades tonight and Sunday, starting first in
the south Washington Cascades then spreading southward through
Sunday. There will be decent valley rains with it as well. The snow
level on Sunday will lower into the coastal mountains and the Cascade
foothills Sunday afternoon as the front passes through, but
precipitation will be dwindling so any accumulations will be limited.

The next system will be dropping in from the northwest later Sunday
night and Monday. Look for additional significant snows in the
Cascades with this system, perhaps 6 inches or a bit more. Drying
overnight Sunday night behind the cold frontal passage of Sunday will
lead to a wet bulb cooling situation over our forecast area that
could drop snow levels close to the valley floor by early Monday. NAM
and GFS model soundings still support this possibility. At this point
expect a rain or snow mix to develop after midnight Sunday night that
could change to mainly snow through a good part of the morning
Monday. Roads will be warm so the snow may take a while to stick and
accumulate, but close to an inch is within the range of
possibilities. The best chance of accumulations will be from around
Salem northward as the precipitation will be starting later in the
south and mix over to rain a bit faster.

The Cascade foothills and the coastal mountains will also see some
snow accumulations with the late Sunday night and Monday system,
Accumulations could reach 1 to 3 or 1 to 4 inches.

A light low level southerly flow should eventually lift the snow
level above the valley floors in the afternoon but it may still hover
in that 1000 foot range. These exact details are still not certain,
it just looks as though this is the most likely scenario at this
point.

The models hold onto a bit of energy from the Monday system into
Monday night as a trailing surface low lingers near the coast. Have
upper PoPs for that time frame. Nighttime cooling will lead to snow
levels lowering back down near the valley floor.

Look for drying from the north Tuesday as the remains of this system
move south, though a few snow showers could linger into Tuesday
morning, especially south of Salem. Tolleson

.LONG TERM...Tuesday night through Saturday...An upper level ridge
will move across the Pac NW late Tuesday/early Wednesday ushering in
a brief period of dry weather. With cloud cover decreasing slightly
overnight we could see the coldest temps of the season thus far,
with temps at the coast near 30. Temps in the interior valley will
generally be in the upper 20s, with temps in the Cascades in the low
teens. In addition to the cold temps, residual surface moisture
combined with clearing skies could provide ideal conditions for
radiational fog so decided to add the mention of freezing fog for
much of the interior valley. However, brisk east winds near the
Gorge should prevent any fog development. 

Unsettled winter weather looks to return by late Wednesday. The GFS
and ECMWF are starting to slowly converge on one another, but the
GFS remains the more aggressive solution. Both models continues to
suggest that moisture will initially overrun the cold air in place.
Model soundings show a brief period of valley snow will be possible
at the beginning. However, as the warm front lifts north Thursday
morning expect valley snow to transition to a mix of snow, freezing
rain, or rain before turning to all rain later in the day. However,
there is still a lot of uncertainty with this system, and the exact
threats and impacts will be influenced by how cold the surface gets
over the coming days.

Active weather looks to continue on Friday and Saturday as a series
of shortwaves move across the region. Both models show a warmer air
mass over the area, with snow levels generally around 3500 ft. As
such, the threat of valley snow will be minimal late next week. /64
&&

.AVIATION...Have continued to see a mix of VFR with periodic MVFR
Cigs across the region this afternoon. Showers have begin to drift
off the coast range the last couple hours and will reinforce the
frequency of cigs 015-025 through about 01z and mainly north of
KSLE.

A cold front will drop from the north later this evening bringing
MVFR cigs to the terminals 03-06z along with the chance for IFR
Vsbys to the coast. Later the front arrives and expect IFR Cigs
along with IFR Vsbys at the coast and MVFR inland.

KPDX AND APPROACHES...Will see occasional cigs 020-025 in the
terminal area as showers drift ENE from off the coast range.
Expect showers and lower cig threat to diminish for a few hours
this evening beginning 01z. After 05z, Cigs will lower from the
approaching cold front. Cigs eventually drop to 015 later
overnight as rain intensifies. Feel cigs could break up or lift to
VFR as early as 14z behind the cold front but will more likely
wait until after 17z. /JBonk
&&

.MARINE...Offshore swell this afternoon has been slow to decay and
was forced to extend the Surf advisory through this afternoon. Now
have concerns it may need to be extended again for a couple more
hours, at least for the Central Coast. Seas remain the mid teens
through tonight.

Winds should remain below small craft advisory criteria
through tonight. Then a trough will ride east along the frontal
boundary late tonight into Sun AM. Models continue to be split as
to if this frontal band will develop into a closed low as it moves
south. The last few times this forecaster has seen the GFS/NAM
create the closed low scenario it has either been very weak or
failed to develop at all. As such, feel that keeping the previous
forecast with small craft advisory level winds is the most likely
result. However, if the GFS/NAM (and by extension, the HRRR)
solution is correct...then there could be a 3-5 period of gale
gusts, perhaps touching on 40 kt, south of Tillamook head starting
near 1 am tonight. Unfortunately, the track of this possible low
will miss all buoys with maybe the evening ASCAT passes being the
only possible method of detection later this evening.

A cold front dropping down from the north will bring another
round of small craft winds Sun night into Mon morning followed by
a short period of offshore flow Tuesday. There is potential for a
much stronger and colder system Wednesday night and Thursday.

Seas appear likely to stay in the low to mid teens through Monday
before subsiding below 10 ft Wed. Seas increase again Thursday
possible peaking close to 20 feet again Thursday night. JBonk/Pyle
&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory until 4 PM PST this afternoon for Central 
     Oregon Coast-North Oregon Coast.

     Winter Weather Advisory from 1 AM to 4 PM PST Sunday for 
     Northern Oregon Cascades.

     Winter Weather Advisory from 4 AM to 4 PM PST Sunday for 
     Cascades in Lane County.

WA...Winter Weather Advisory from 6 PM this evening to 10 AM PST 
     Sunday for South Washington Cascades.

PZ...Small Craft Advisory for winds from 3 AM to 10 PM PST Sunday 
     for Coastal Waters from Cape Shoalwater WA to Florence OR 
     out 60 nm.

     Small Craft Advisory for hazardous seas until 4 PM PST Tuesday 
     for Coastal Waters from Cape Shoalwater WA to Florence OR 
     out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 4 PM 
     PST Sunday.

&&

$$

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.

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Western Regional Climate Center, wrcc@dri.edu